PLATE LXXV. 
AcCROTRETA RUDIS Walcott (p. 702). 
[Pl. LXV, fig. 3f.] 
Ficure 1. Enlargement by photography of a small piece of arenaceous shale on which the casts of three ventral and 
three dorsal valves have been compressed. The ventral valves are crushed and spread out toward 
the aperture. Locality 121, Middle Cambrian Rogersville shale, road just east of Harlan Knob, 
4 miles (6.4 km.) northeast of Rogersville [Keith, 1905, areal geology sheet], Hawkins County, Ten- 
nessee. U.S. Nat. Mus. Cat. No. 52112a. 
AcCROTRETA CoNULA Walcott (p. 681). 
Figure 2. Enlargement by photography of a fragment of shale studded with shells of this species. Locality 310a, 
Upper Cambrian shales of Olenus truncatus zone, on Oeland Island, Sweden. U.S. Nat. Mus. Cat. 
No. 35267a. 
2a. Cast of interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 35267a. 
2b. A vertically compressed ventral valve, the type specimen. U.S. Nat. Mus. Cat. No. 35267a. 
Figure 2a is drawn from the specimen which is partly cut by the lower margin of figure 2, just to the left of the 
center. Figure 2b is of a specimen from the same slab of shale but outside the limits of figure 2. 
ACROTHYRA PROAVIA (Matthew) (p. 717). 
[Pl. LXXVI, figs. 3, 3a-u.] 
Fieurr 3. Enlargement by photography of a small piece of limestone in which numerous ventral and dorsal valves 
are embedded. The varying shape of the ventral valves is well illustrated. Locality 10q, Middle 
Cambrian sandstone below the waterfall in Division E2b of Matthew’s [1903, p. 21] Etcheminian, 
Dugald Brook, Indian River, eastern Cape Breton, Nova Scotia. U.S. Nat. Mus. Cat. No. 52052r. 
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